1. Field of the Invention
The present invention relates to a sheet feeder for transporting sheets, an original document transport device which includes the sheet feeder, and an image forming apparatus, such as a copying machine, a multifunction peripheral, or a facsimile machine. The present invention also relates to a sheet feeding method for the sheet feeder.
2. Description of Related Art
In general, an image forming apparatus that handles sheets, such as a copying machine, a multifunction peripheral, or a facsimile machine, is provided with a sheet feeder for receiving sheets and sending out the sheets as necessary. For example, the sheet feeder is disposed inside an original document transport device for automatically and successively feeding original documents (sheets) one by one to a reading position of an image reading part which reads the original document and generates image data. However, when sheets are stacked exceeding a sheet stacking capacity, the sheet feeder may be jammed with sheets, and hence sheet feeding operation cannot be performed appropriately. Further, the sheets may be damaged by being folded or the like. In view of this, some sheet feeders do not perform sheet feeding operation when too many sheets are stacked thereon.
For example, there is known a sheet feeder which includes: a sheet stacking part; a sheet feeding mechanism for feeding sheets one by one from the sheet stacking part; a device casing for supporting the sheet stacking part so as to be movable in a vertical direction; a driving mechanism for moving the sheet stacking part in the vertical direction; sheet stacking detection means for detecting stacking of sheets on the sheet stacking part; sheet position detection means for detecting that an uppermost sheet is positioned in a sheet feedable position, when the sheet stacking part is moved in an upward direction; and control means for determining that a sheet stacking amount is exceeded when the sheet stacking detection means and the sheet position detection means are each in a detection state before the sheet stacking part is moved in the upward direction, in which the control means prohibits operations of the sheet feeding mechanism and the driving mechanism when it is determined that the sheet stacking amount is exceeded. This configuration is intended for avoiding sheet feeding trouble.
For example, an original document transport device, which is a kind of the sheet feeder, feeds original documents as sheets. The original document transport device may include an original document feeding roller which comes into contact with an uppermost sheet of the original documents placed on the sheet placing table. The original document feeding roller is configured to be movable in a vertical direction, for example, so as not to interfere with the placement of original documents when an original document stack (sheet stack) including a large number of sheets are placed. Further, an upper limit detection sensor may be provided for detecting that the original document feeding roller has reached an upper limit position.
Here, there may be a case where, for example, an original document stack to be placed on the sheet placing table slightly exceeds a loadable thickness (maximum thickness capable of being transported without causing a non-feed jam). In such a case, the original document stack may be placed on the sheet placing table, but may be sandwiched between the original document feeding roller and the sheet placing table because the original document feeding roller is capable of moving upward. In this state, the original document feeding roller may fail to rotate, for example, and hence the original documents cannot be sent out appropriately, which leads to a non-feed jam.
In view of the above, as in the known sheet feeder, the feeding of original documents may be stopped without exception, when the upper limit detection sensor (sheet position detection means) detects that the original document feeding roller is in the upper limit position when original documents are set. When such a control is performed, the occurrence of a non-feed jam may certainly be prevented.
However, depending on the configuration and the mounting position of the upper limit sensor, the upper limit sensor may sometimes detect that the original document feeding roller is in the upper limit position even when the original document feeding roller has not reached the upper limit position completely. In other words, an allowance may be made in the detection performed by the upper limit sensor. Accordingly, even when the upper limit sensor detects that the original document feeding roller is in the upper limit position when original documents are set, a non-feed jam does not necessarily occur. That is, even when the upper limit sensor has detected that the original document feeding roller has reached the upper limit position, feeding operation of sheets or original documents may still be performed without causing a non-feed jam even when several more sheets or original documents are additionally placed thereon.
As described above, in a configuration where the sheet feeding operation is stopped without exception when the upper limit sensor has detected that the sheet feeding roller has reached the upper limit position, there arises a problem that the thickness (number of sheets or original documents) of the sheet stack or of the original document stack to be loaded at one time is reduced as compared with the thickness (number of sheets or original documents) of the sheet stack or of the original document stack that may originally be loadable in the device.